The present invention relates to a method and system for aligning an ultraviolet beam and an optical fiber during the manufacture of an optical filter in the optical fiber. Optical filters comprising Bragg gratings formed in optical fibers are useful for dispersion compensation in optical communication systems, and for various other applications. A known method of manufacturing such filters illuminates a photosensitive optical fiber with ultraviolet light through a phase mask, thereby imprinting a modulation pattern in the refractive index of the fiber core. The manufacturing process is difficult, because the optical characteristics of the filter are extremely sensitive to the degree of index modulation. Unintended variations in the degree of index modulation can cause Fabry-Perot resonance and undesired chirping, leading to such problems as ripple in the reflection band, generally reduced reflectivity in the reflection band, and prominent sidelobes outside the reflection band.
The filter characteristics also depend on the length of the in-fiber Bragg grating, and some applications require fairly long gratings. Dispersion compensation on the order of one thousand four hundred picoseconds per nanometer over a bandwidth of seven nanometers, for example, requires an in-fiber Bragg grating with a length of one to two meters.
Since the degree of index modulation depends on the amount of ultraviolet illumination received by the fiber core, highly uniform illumination is a key consideration in the manufacturing process. The source of the ultraviolet light is a laser apparatus. Uniform illumination requires the use of a highly stable source, such as an argon ion laser, which emits a comparatively weak and narrow ultraviolet beam by second harmonic generation. (The comparison is with a krypton-fluoride or KrF laser. A KrF laser emits a larger and stronger beam, but the beam intensity and plane of polarization tend to fluctuate over time.) The intensity of an argon ion laser beam can be increased by focusing the beam, but the beam then becomes even narrower. This narrow beam must be scanned along the length of the optical fiber, and for uniform illumination, the center of the beam must be kept accurately aligned on the fiber core throughout the scan. Accurate alignment of a narrow beam on a thin optical fiber is no simple matter, particularly when a long in-fiber grating is being manufactured.
A known method of alignment is to monitor visible light produced by fluorescence when the core of the optical fiber is illuminated by the ultraviolet beam. The fluorescence arises from the germanium doping that makes the core photosensitive. The fluorescence is measured by an optical power meter connected to one end of the optical fiber, and the measured value is fed back to a positioning system that positions the fiber under the beam. The feedback control system operates to keep the fiber in the position that produces maximum fluorescence, this being the position in which the beam is centered on the fiber core.
This fluorescence monitoring method of alignment leaves room for improvement, however, because while a decrease in the measured optical power may indicate that the beam is drifting away from the fiber axis, the decrease does not indicate the direction of the drift. The positioning system must move the fiber in one direction, and determine whether this movement produces an increase or a further decrease in the measured optical power. If a further decrease is detected, then the positioning system has moved the fiber in the wrong direction, and must now move the fiber in the opposite direction.
Natural variations in the amount of fluorescence also make it difficult to determine when the maximum optical power has been obtained. Even when the ultraviolet beam is held stationary on the same spot on the optical fiber, the intensity of the fluorescence varies considerably over time, not reaching a constant level until about thirty seconds have elapsed. One consequence is that the measured optical power depends on the scanning rate, which may be varied intentionally to create an apodized Bragg grating.
For these reasons, a feedback control system targeted at maximum fluorescence requires complex control circuitry. Moreover, even with complex control circuitry, the beam alignment has a tendency to wander, due to natural fluorescence variations and wrong-direction control. With a sufficiently wide beam, such wandering could be tolerated, but the optical filter manufacturer would prefer to be able to focus the beam as tightly as possible, thereby making the beam as intense as possible, so that the scanning process can be completed as quickly as possible.